How can you stop live video from playing in the GUI?

Hi, I have code that displays video footage from a camera in a GUI. I am unable to stop this video once it starts, the code that I copied to get the image to display tells the user to close the image simply by clicking on the "x" button on the top right corner. The problem with this, is that the "x" box is not visible once the image runs inside my GUI.

Most attempts either are ignored or cause matlab to freeze/hang. I believe a loop is running when I try to close the video and it simply ignores the close request.

disp('iCam Live Mode Preview')
PrepareAcquisition()
% init system
disp('Initialising Camera');
ret=AndorInitialize('');
CheckError(ret);

disp('Configuring Acquisition');
[ret]=CoolerON();                             %   Turn on temperature cooler
CheckWarning(ret);
[ret]=SetAcquisitionMode(5);                  %   Set acquisition mode; 5 for RTA
CheckWarning(ret);
[ret]=SetExposureTime(0.02);                  %   Set exposure time in second
CheckWarning(ret);
[ret]=SetReadMode(4);                         %   Set read mode; 4 for Image
CheckWarning(ret);
[ret]=SetTriggerMode(10);                     %   Set Software trigger mode
useSoftwareTrigger = true;

if ret == atmcd.DRV_INVALID_TRIGGER_MODE
    disp('Software trigger not available, using Internal trigger instead')
    SetTriggerMode(0);                        %   Set internal trigger mode
    useSoftwareTrigger = false;
end

CheckWarning(ret);
[ret]=SetShutter(1, 1, 0, 0);                 %   Open Shutter
CheckWarning(ret);
[ret,XPixels, YPixels]=GetDetector;           %   Get the CCD size
CheckWarning(ret);
[ret]=SetImage(1, 1, 1, XPixels, 1, YPixels); %   Set the image size
CheckWarning(ret);

disp('Starting Acquisition');
[ret] = StartAcquisition();
CheckWarning(ret);

I = zeros(YPixels,XPixels);
h = imagesc(I);
colormap(gray);

warndlg('To Abort the acquisition close the image display.','Starting Acquisition');
while(get(0,'CurrentFigure'))

       if useSoftwareTrigger == true
            [ret] = SendSoftwareTrigger();
                 CheckWarning(ret);

                    [ret] = WaitForAcquisition();
                    CheckWarning(ret);
       end

       [ret, imageData] = GetMostRecentImage(XPixels * YPixels);
      CheckWarning(ret);

       if ret == atmcd.DRV_SUCCESS
           %display the acquired image
            I=flipdim(transpose(reshape(imageData, XPixels, YPixels)),1);
              set(h,'CData',I);
              drawnow;
       end
  end

disp('Acquisition Complete! Cleaning Up and Shutting Down');
[ret]=AbortAcquisition;
CheckWarning(ret);
[ret]=SetShutter(1, 2, 1, 1);
CheckWarning(ret);
[ret]=AndorShutDown;
CheckWarning(ret);

I tried to use the last 6 or 7 lines of this code as a separate "stop" button but this occurs once pressed, most of the time I cant even open the main matlab window to see due to it hanging...

 Can someone explain how I can close or stop this video without matlab crashing?

Thanks Johnny

Answer 1

[Geoff Hayes]

Johnny - where is the above code taken from? Is it within a callback (push button?) from your GUI that kicks off the process of capturing images and displaying them in a figure, h, that is outside of your GUI? Or is it (more likely) that you are displaying the images within an axes widget on your GUI - which would explain why there is no x button to close.

So let's assume that you have start button that invokes the above code and starts the image acquisition, and that you have a stop button that will try to stop the process.

It is almost correct when you say that since the while loop is running, all other requests (like pressing the stop button) will be ignored. "Almost" because you have a drawnow command that should allow other processes to interrupt the current one (see callback sequencing and interruption for details). So pressing a stop button should allow us to exit the while loop if we change a couple of things. I'm assuming that the interruptible property of the start button is on (which is the default setting).

In the above code, just before the while loop starts add the following lines and change the condition in the loop

 % create a data field within handles to say that we've started the
 % image acquisition
 handles.imgAcqStarted = 1;

 % save the field to the handles structure
 guidata(hObject,handles);

 % condition on this field being one
 while handles.imgAcqStarted==1

      % get the most recent copy of handles
       handles = guidata(hObject);

      % etc., as before
 end

So we will continue to do the image acquisition until told otherwise. We need to get the updated copy of handles at each iteration because it is the stop button that will update this object, and in particular the imgAcqStarted flag. In the stop button callback do

 function stop_Callback(hObject, eventdata, handles)

 fprintf('trying to stop...\n');

 % stop the image acquisition
 handles.imgAcqStarted = 0;

 % save the field to the handles structure
 guidata(hObject,handles);

So when the stop button is pressed, the above code should be invoked. The imgAcqStarted will now be zero, and when the other callback resumes, the while loop condition should evaluate to false, and we will exit the loop.

The above code should help provided that we always call the drawnow function. However, this isn't always the case as it is only called when ret == atmcd.DRV_SUCCESS. So we will need to handle the case where this isn't a success. Just change this block of code to

 if ret == atmcd.DRV_SUCCESS
     %display the acquired image
     I=flipdim(transpose(reshape(imageData, XPixels, YPixels)),1);
     set(h,'CData',I);
     drawnow;
 else
     pause(0.001);
 end

Adding a short pause will allow this callback to be interrupted by the stop button callback, and so the image acquisition should end gracefully.

One "gotcha" that I can think of is what happens if the user closes the GUI (hits the x). How do we instruct the start button callback to stop? We can do something similar to the stop button, but it does get a little kludgy. For your GUI/figure, create a CloseRequestFcn callback and add the following code (in this case, my GUI/figure name (or tag) is figure1)

 % --- Executes when user attempts to close figure1.
 function figure1_CloseRequestFcn(hObject, eventdata, handles)
 % hObject    handle to figure1 (see GCBO)
 % eventdata  reserved - to be defined in a future version of MATLAB
 % handles    structure with handles and user data (see GUIDATA)

 if handles.imgAcqStarted==1
    handles.imgAcqStarted = 2;
    guidata(hObject,handles);
 else
    delete(hObject);
 end

In the above, if the image acquisition has started (so its integer value is one) then we set this flag to two (so that we exit the while loop) and save the change to the handles structure. If image acquisition has not been started, then we just delete the figure and it will close gracefully.

So if the flag/indicator changes to two, then the start button callback resumes and will exit the while loop since this flag is no longer one. The acquisition will end gracefully but we still need to close the figure. Add the following line to the end of your start button callback

 if handles.imgAcqStarted==2
    delete(handles.figure1);
 end

This should then delete the GUI/figure, hopefully without errors. If I knew more about how the image acquisition worked, I would suggest replacing some of the above with a timer that would periodically fire and grab the next image. You may still be able to do this, but I'm not sure how often you wish to acquire new images and whether a timer can be fast enough.

Try the above and see what happens!

[Jonathan O'Neill]

Hi Geoff

The code was taken from an "example preview image" file left by a previous student who used to work here, I'm a recent graduate but my Matlab experience is pretty limited. Yes the image is being viewed in an axes on my GUI, once I created the axes it seemed to automatically decide to display there... which I am happy enough with.

I have tried this code but once I click start I get this error:

Initialising Camera
Configuring Acquisition

    Undefined function or variable "imageData".
    Error in CamTestGUI>previmage_Callback (line 1671)
        I=flipdim(transpose(reshape(imageData, XPixels,
        YPixels)),1);
    Error in gui_mainfcn (line 96)
            feval(varargin{:});
        Error in CamTestGUI (line 42)
            gui_mainfcn(gui_State, varargin{:});    Error in
        @(hObject,eventdata)CamTestGUI('previmage_Callback',hObject,eventdata,guidata(hObject))    Error while evaluating uicontrol Callback

[Geoff Hayes]

Hi Johnny - I'm guessing that the above code (in your comment) is the same code from your questions and so it all appears in the start button callback, or previmage_Callback? How does this callback called i.e. what does it respond to? imageData should be defined provided that you left the code as

[ret, imageData] = GetMostRecentImage(XPixels * YPixels);
CheckWarning(ret);
if ret == atmcd.DRV_SUCCESS
   %display the acquired image
   I=flipdim(transpose(reshape(imageData, XPixels, YPixels)),1);
   set(h,'CData',I);
   drawnow;
else
   pause(0.001);
end

If that is not the case, can you post/attach your code?

[Jonathan O'Neill]

Yes Geoff its all in the previmage_Callback. Thanks for the help, and sorry for the late reply. I have somehow managed to destroy my code for the GUI and am rebuilding it at the minute (oops). I should have it sorted soon but I think your answer should work out, maybe my code was so messed up it was affecting this. The previmage callback is activated once I click on a button to start the video. I will let you know if this code works soon, thanks!

[Jonathan O'Neill]

Thanks Geoff your code did the trick, all is good now!

[Geoff Hayes]

Awesome!